Process of producing alloys



Patented June 13, 1935 UNITED STATES PATENT OFFICE No Drawing.Application August 15, 1931, Serial No. 557,371

6 Claims. (Cl. 75-45) This invention relates to the production of alloyirons and steels of low silicon content, especially rustless irons andsteels, and more particularly to such irons and steels wherein the alloycontent thereof is derived by a direct reduction of a reducible oxide ofthe alloy using a silicon containing reducing agent.

One of the objects of my invention is the production in a simple,practical and economical manner, of alloy irons and steels, such aschromium, manganese, vanadium, tungsten, and like irons and steels(wherein the alloy content is high and the carbon, silicon and the likecontent is low) from readily available and inexpensive raw materialsand/or alloy scrap, employing and familiar operating equipment.

I known Another object is the highly eflicient and thoroughly reliableelimination of silicon from fer- 0 rous alloys of the class indicated,of objectionably high silicon content and desirably high alloy contentwithout undue loss of such readily oxidizable alloy content.

pointed out hereinafter.

The invention accordingly consists in the several steps and in therelation of each of the same 5 to one or more of the others as describedherein and the scope of the application of which is indicated in thefollowing claims.

As conducive to a clearer understanding of certain features of myinvention it may at this D point be noted that in heretofore knownand/or used processes of producing alloy irons and steels, andespecially rustless or high chromium irons and steels, wherein the alloycontent is obtained by a direct reduction of an oxide or ore of 5 thealloy ingredient into the ferrous metal, a silicon containing reducingagent is generallyemployed due to the commercial availability,coinparative cheapness and general eificiency of such a reducing agent.

Silicon, however, is readily soluble in the molten ferrous metal in allpropor-' tions. Special precautions, therefore, need be observed tocontrol or limit the amount of silicon that may enter the metal in thesuccessful prac-' tice of such a process.

In heretoforeknown processes,

one manner for 'ficiency and economy of production are measpi'eventingsilicon contamination is to utilize a substantial excess of reducibleoxide in relation to the silicon containing reducing agent. By thismeans, if the technique of the process is carefully governed, siliconcontamination of the metal bath to an undesirable extent may beprevented. To effectively employ such a means, however, it is necessaryto maintain an excess of reducible oxide of sixty per cent, or more, inorder that the silicon content of the product may be reliably mainmtained below .50 per cent. This method, however, gives but a low alloyrecovery from the reducible oxide.

In heretofore known processes where the excess of reducible oxide isdecreased materially below sixty per cent, the silicon content of theproduct proportionately increases. For example, where the excess ofreducible oxide is decreased to forty per cent the silicon contentbecomes of the order of two per cent or more. Such a high siliconcontent is objectionable for most purposes and heretofore known attemptsof decreasing this silicon content any appreciable extent results inundue loss of the readily oxidizable and expensive alloy elements.

One of the outstanding Objects of this invention is the production ofalloy irons and steels of low silicon content and a desired high alloycontent by direct reduction of the alloy ingredient from a reducibleoxide thereof wherein the quantities 'of raw materials employed aredecreased over heretofore known and/or used processes of the sortindicated and wherein the efureably increased.

Referring now to the practice of my invention, in the production of aferrous alloy of low silicon content from available and comparative-. lyinexpensive raw materials, a bath of ferrous metal is prepared in asuitable furnace. Conveniently, ordinary plain low carbon steel scrap ischarged into' an electric furnace and melted down therein together withsuitable amounts of aniron oxideand a basic slag forming agent inaccordance with the usual practice so as to produce a bath of ferrousmetal of low carbon content.- The slag formed during this period ofinitial melt down and carbon elimination is preferably removed prior tothe. next step wherein the alloy element is added to the-bath. I

' To economically produce a ferrous alloy of the class indicated frominexpensive raw materials,

a reducible oxide of a desired alloy ingredient,

ordinarily an ore of the alloy, is charged'into the bath of ferrousmetal. With this oxide.

there is charged an appropriate amount of reducing agent. A siliconcontaining reducing agent and preferably a fifty per cent ferro-silicon.due to the comparative cheapness, efficacy and availability of thismaterial is ordinarily preferred.

In accordance with the provisions of my invention the proportion ofreducible oxide in excess of the chemical equivalent of reducing agentby forty per cent or less. This decreased amount of excess oxide overheretofore known methods wherein an excess of reducible oxide isemployed to avoid undue silicon contamination of the product permits adirect saving in raw materials, allows supplementary economies inhandling the material and is conducive to improved furnace operationduring the reduction period and increased rapidity of operation.

Incident to the reduction of the oxide of the alloy element, siliconfrom the reducing agent is directly absorbed by the bath of molten metalsince, as indicatedabove, silicon is readily ,soluble in molten iron.

To free the metal from the objectionable quantity of silicon thusintroduced, an oxidizing slag comprising a mixture of an iron oxidetogether with a quantity of a basic slag forming material such ascalcined lime sufficient to assure fluidity of the slag, is thenprepared on the surface of the ferrous alloy bath. A minimum amount ofslag and a slag of maximum eificiency is assured by first freeing thesurface of the bath of molten metal from the slag remaining afterreduction period described above.

The concomitant objectionable oxidation of the added alloy ingredient isprevented by maintaining the bath of metal at a temperature favorable tothe oxidation of the objectionable silicon but unfavorable or at leastconsiderably less favorable to the attendant oxidation of the alloymetal. When the silicon content is brought down to the desired low valuethis stage of the process is complete. The oxidizing slag is thenremoved and the metal is refined and finished for pouring in the usualmanner.

As illustrative of the practice of my invention in the production ofhigh chromium or, rustless, iron and steel of about seventeen per centchromium, 0.10 per cent carbon and about 0.50 per cent silicon, about5,000 pounds of ordinary steel scrap is melted down in a six ton Heroultelectric furnace. Chrome ore together with a. fifty per centferro-silicon is then charged into the furnace in such quantities thatupon reduction of the ore the desired alloy content of the metal isachieved.

When a typical chrome ore analyzing fortyeight per cent CrzO: andfifteen per cent FeO is used about seventeen per cent by weight ofsilicon is theoretically required to reduce all the available chromiumand iron in this ore. Thus, for example, one hundred pounds of siliconare chemically equivalent to about six hundred pounds of ore and twohundred pounds of a fifty per cent ferro-silicon are equivalent to sixhundred pounds of chrome ore.

In order that excessively high silicon contamination may be preventedand so that a preponderant excess of ore may be avoided an excess ofchrome ore of about forty per cent above that theoretically required ispreferably employed. Thus eight-hundred and forty pounds of chrome oreare used in proportion to one hundred pounds of silicon or two hundredpounds of a fifty per cent ferro-silicon.

For a six ton heat of metal, about ten thousand pounds of chrome ore andtwo thousand five hundred pounds of fifty per cent ferro-sllicon arecharged into the furnace after the melt down of the five thousand poundsof ordinary steel scrap as indicated above.

As a convenient outlet for the large quantity of chromium steel scrapwhich is readily available, about four thousand pounds of this chromiumsteel scrap is charged into the furnace along with the chrome ore andferro-silicon. Although alloy scrap analyzing about the same in chromiumand carbon as desired in the final product is preferably used whereconvenient, good results are achieved where the alloy scrap is high inalloy content and low in carbon. The alloy scrap readily melts down andenters the bath of ferrous metal, thus directly supplying chromium andiron with the introduction of but a minimum of impurities.

Under the action of the intense heat of the electric furnace and thedirect contact with the hot bath of ferrous metal an action between thereducible oxide, illustratively chrome ore, and the reducing agent,illustratively fifty per cent ferrosilicon, immediately sets in andrapidly progresses. The chrome ore is readily reduced and the resultantmetals, chromium and iron, directly enter the bath beneeath. Incident tothe reduction'of the reducible oxide a thick slag forms on the bath ofmetal and from time to time this slag is conveniently removed.

After the action between the reducible oxide and the reducing agent isat an end the reduction period is complete. At this stage of the processthe bath of molten metal has a chromium content of about 17.5 per cent,a carbon content under 0.10 per cent, and a silicon content of about1.75 per cent.

While the chromium and carbon contents are about the values desired inthe final product, the silicon content is objectionably high. Inaccordance with the further provisions of my invention all slagremaining on the bath of metal after the completion of the abovedescribed reduction period is removed from the surface of this bath,leaving the metal free of slag and exposed to the furnace atmosphere.

The electric power supplied the furnace is then cut off and the furnaceelectrodes are raised out of contact with the bath of metal. With thefurnance chamber thus unobstructed there isadded to the surface of thebath of ferrous alloy a mixture of about two hundred pounds ofmagnetically concentrated iron ore and about two hundred pounds ofcalcined lime for the ingredients and proportions illustratively setforth above. This mixture of ingredients quickly spreads over the entireexposed surface of the bath of molten metal.

The furnace electrodes are now lowered and the electric power turned onso that arcs are established above the loose mixture on the surface ofthe bath. Under the heating action of the furnace the iron ore and limereadily fuse to form a highly active oxidizing slag which attacks thesilicon content of the bath. The slag in addition to serving as anoxidizing agent for the silicon content of the bath also serves toprotect the bath of metal from carbon contamination from the furnaceelectrodes during the further operations.

To increase the fluidity of this slag and render it strongly basic andaccelerate the attack on silicon there is preferably added a furtheramount of about two hundred and fifty pounds of calcined lime as soon asthe mixture commences to fuse.

The total quantity of active oxidizing agent such as iron ore and/ormill scale, together with dred pounds and three hundred-pounds and theburnt lime between one hundred pounds and six hundred pounds.Theparticular amount of lime employed isgoverned by the character of theslag formedii Just sufficient lime is used as is required to give a dry,limey, or basic slag, 'a slag which is especially conducive to theelimination of silicon from the bath of metal. Where insufilcient limeis used the slag is acid in characterand the silicon content of the bathis not readily oxidized.

While this oxidizing slag readily attacks the silicon absorbed by thebath of metal from the silicon containing reducing agent during thereduction stage of the process and thus decreases the total siliconcontent of the bath, it also attacks the alloy metal, illustrativelychromium. The loss of chromium through oxidation is highly undesirablesince there would be required the initial introduction of an excess ofchromium to compensate for this later loss, all at a considerableincreased cost.

To achieve efflcient oxidation of silicon contained in the bath and yetprevent undue loss of the chromium, the temperature of the bath ismaintained within fairly narrow limits. Where the temperature of thebath is allowed to rise much above 3000 F. the attack on the chromium isaccelerated, and, where the temperature is allowed to fall much below2800 F. all action on both the contained chromium and silicon is veryslow. Dm'ing this stage of the process wherein the silicon content ofthe bath is brought down to a desired low value the temperature of thebath of metal is thus preferably maintained between 2800" F. and 3000 F.

Under the action of the actively oxidizing slag the silicon content ofthe metalis lowered to about 0.5 per cent in about forty-five minutes,for the conditions illustratively set forth above, accompanied by anegligible loss of chromium.

After the elimination of silicon has sufilciently progressed and thesilicon content of the bath is brought to a desired low value, theoxidizing slag is preferably completely removed from the surface of thebath and a refining slag made up of about four hundred fifty pounds ofcalcined lime, seventyfive pounds of fiuorspar and about seventy-fivepounds of fine ferro-silicon is added in accordance with the usualpractice. This refining slag promotes deoxidation of dissolved oxides ofiron, chromium, and silicon that are dissipated throughout the metal.When sufficient time for refining has elapsed the metal is tapped intosuitable molds. Upon analysis the heat of metal is found to containabout seventeen per cent chmmium. from 0.30 per cent to 0.60 per centsilicon, and less than 0.10 per cent carbon.

It will be seen that there has been provided in this invention an art inwhich the various objects hereinbefore noted, together with manythoroughly practical results, are successfully achieved.

Although as illustrative of my invention undesirable silicon iseliminated from a bath of alloy iron or steel wherein the alloy contentof the bath is obtained by a direct reduction of a reducible oxide ofthe alloy by means of a silicon containing reducing agent, it will beunderstood that silicon may be eliminated from a bath of ferrous alloymetal of objectionally high silicon content however produced; as forexample, by the melt down of ferrous alloy scrap of objectionablesilicon content in, for example, an induction type of electric furnace.I

And, although in the illustrative embodiment of my invention, theproduction of a high chromium iron orsteel of low. silicon content isspecifically described it will be understood that high manganese,vanadium, tungstenand like alloy irons and steels, or combinations ofthe same, of low silicon contents may be achieved by a direct reductionof one or more ores or oxides of the alloy metals with a siliconreducing agent, obtaining improved recovery over heretofore knownmethods, and thereafter selectively oxidizing the excess silicon contentof the iron or steel by maintaining the metal under basic oxidizing slagconditions at a temperature favorable to the oxidation of silicon butunfavorable to the oxidation of the desired alloy constituent all asmore particularly described above.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiments hereinbefore set forth it is tobe understood that all matter described herein is to be interpreted asillustrative and not in a limiting sense.

What I claim is:

1. In the production of rustless irons and steels, the method ofeliminating silicon from a bath of iron or steel having a highpercentage of chromium and an objectionable amount of silicon whichcomprises, forming thereon a basic iron oxide slag substantially freefrom oxides of chromium, and maintaining the temperature of the bathbetween 2800 F. and 3000 F., whereby the silicon contained in said bathis selectively oxidized thereby achieving ironand steel high in chromiumand'substantially free of silicon and sub stantially free from chromiumoxide inclusions.

2. In the production of alloy irons and steels high in a desnedoxidizable alloy and low in sili- .con, the art which includes, reducingan ore of the desired alloying ingredient with a silicon-containingreducing agent in the presence of a ferrous metal bath, the amount ofsaid ore being chemically in excess of said reducing agent by less than40 'per cent, thereby achieving a bath of ferrous metal high in thedesired alloying ingredient and objectionably high in silicon covered bya slag, removing the slag overlying said bath of metal, forming on saidbath a basic oxidizing slag of iron oxide and burnt lime, andmaintaining said bath and overlying oxidizing slag at a temperaturefavorable to the oxidation of silicon from said bath but relativelyunfavorable to the oxidation of the desired alloying ingredientcontained therein, whereby iron or steel of a de-- high in chromium andobjectionably high in silicon covered by a slag from which chromium issubstantially completely recovered, withdrawing said slag from thesurface of said bath and forming thereon a basic oxidizing slag of ironoxide and burnt lime greatly in excess of the silicon present, andmaintaining said bath and secondmentioned slag at a temperature fromabout 2800 F. to 3000 F., thereby selectively oxidizing the siliconpresent in said bath with a minimum oxidation of the chromium present.

4. In the production of rustless iron of low silicon content, the artwhich comprises, preparing a ferrous metal bath; adding to said bath amixture of a reducible oxide of chromium and ferrosilicon wherein saidoxide is in excess of the chemical equivalent of said ferrosilicon byless than 40 per cent, whereby said oxide is reduced, the chromium metalentering the bath together with silicon in an amount greater thandesired in the final product and leaving a slag on the surface of saidbath from which the metallic valuesare largely recovered; removing saidslag to expose the clean metal of the bath and adding thereto iron oxideand lime thus forming a basic oxidizing slag; and maintaining said bathat a temperature favorable to the oxidation of silicon and substantiallyunfavorable to the oxidation of chromium, thereby decreasing the siliconcontent of said bath.

'5. In the production of rustless iron of low silicon content, the artwhich comprises, preparing a ferrous metal bath; adding to said bath amixture of a reducible oxide of chromium and a silicon-containingreducing agent therefor wherein said oxide is in excess of the chemicalequivalent of said reducing agent by forty per cent or less whereby saidoxide is reduced, the chromium metal entering the slag together withabout two per cent of silicon leaving a slag on the surface of saidbath; removing substantially all of said slag to obtain a clean surfaceof said bath and forming thereon a basic slag rich in iron oxide; andmaintaining said bath and slag at a temperature of from 2800 F. to 3000F. whereby the silicon content of the bath is selectively oxidized.

6. In the production of rustless iron of desired chromium content andlow silicon content, the art which includes,- preparing a ferrous metalbath, charging chrome ore and a silicon-containing reducing agent ontothe surface of said bath in proportions such that the ore is in chemicalexcess of the reducing agent by less than 40 per cent whereby the ore isreduced accompanied by a contamination of said bath with silicon,removing the slag overlying said bath from which the metallic valueshave been recovered and forming on said bath a basic oxidizing slag,maintaining said bath and slag at a temperature favorable to theselective oxidation of silicon in the presence of iron and chromiumuntil a desired low silicon content of the bath is reached, and removingsaid desiliconizing slag and forming on the surface of said bath arefining slag of ferrosilicon and lime to free the metal of oxideinclusions and achieve clean rustless iron of a desired low siliconcontent.

WILLIAM BELL ARNESS.

